1. Field of The invention
The invention relates to an optical band-pass filter device for use with the light, and particularly to the filter assembly and the method making the same.
2. The Related Art
Generally speaking, the common light occupies a wide band on its corresponding optical wavelength range. Oppositely, in the optical field, a light having very narrow range wavelength in a pulse-like or a specific manner is desired due to its capability of controlling and expectation. Therefore, an optical band-pass filter device is used to narrow the range of the wavelength of a designated light for achievement of an output light having a pulse-like character with the relation between its wavelength and intensity. Such characterized light has substantially a useful characteristic to be applied to many matters such as testing or transmission in the optical industry.
As well known, one type of optical band-pass filter is the dielectric-coating cavity filter (including single-cavity and multiple-cavity filter) which is a Fabry-Perot interferometer made with thin films. The cavity layer, namely the cavity, is a spacer layer which has an optical thickness equal to one half (or an integral multiple of one half) the wavelength of the light to be transmitted by the filter. The cavity layer is sandwiched between two reflecting quarter-wave stacks which are made up of alternating high and low index layers, each layer having an optical thickness of one fourth of the passband center wavelength. The typical single-cavity filter in the prior art is constructed by coating successively on a substrate a filter-film composed of filter-layers with alternating high and low refraction index as in the following form: EQU (substrate) (HL).sup.n (2H).sup.m (LH).sup.n,
or EQU (substrate) (LH).sup.n (2L).sup.m (HL).sup.n,
wherein H refers to a layer of high index material such as TiO.sub.2, having an optical thickness of one quarter wavelength; and L refers to a layer of low index material such as SiO.sub.2, having an optical thickness of one quarter wavelength. The n and m are integrals indicating the repeated times for the structure in the corresponding parentheses respectively.
The multiple-cavity filter in the prior art is manufactured by successively coating on a substrate a multiple-cavity filter-film of alternating high and low index layers. The multiple-cavity filter-film is composed of several single-cavity filter-films with a coupling layer between every adjacent two single-cavity filter-films. The coupling layer typically has an optical thickness of one fourth of the passband center wavelength plus integrals of quarter center wavelength. It should be pointed out that the multiple-cavity filter is composed by several single-cavity or sub-multiple-cavity filters optically coupled together by the coupling layers, and the shape factor of such a multiple-cavity filter has large difference from that of the composing filters. The shape factor is defined as .DELTA..lambda..sub.0.01 /.DELTA..lambda..sub.0.5, where .DELTA..lambda..sub.0.5 is the bandwidth at 0,5 maximum and .DELTA..lambda..sub.0.01 is the bandwidth at 0.01 maximum. The shape factors of single-, double-, triple- and quadruple-cavity filter typically are 11, 3.5, 2.0 and 1.5. An equivalent filter composed by cascading several optically decoupled single or sub-multiple-cavity filters has its shaped factor approximately equal to the composing filers. Thus there are important differences between these two kinds of filters, and this invention relates to a multiple-cavity optical band-pass filter composed by several single-cavity or sub-multiple-cavity filters optically coupled together by the coupling layers.
The performance of a multiple-cavity dielectric-coating band-pass filter strongly depends on its cavity number. In most applications, the more cavity number there is, the better the performance of the multiple-cavity filter, including higher stop band rejection, more flattened pass-band and faster roll-off speed, as shown in FIGS. 1(A) and 1(B) wherein .lambda. represents the wavelength and I represents the intensity.
In fact, to attain a sufficiently reliable output light, in certain conditions, there are nearly one hundred filter-layers with alternating high and low refraction index to be successively coated on the substrate to form a multiple-cavity filter-film for a filtering purpose. Because the filter-layers are coated unto the substrate one by one, it takes time and costs money to implement the whole complete multiple-cavity filter assembly. Additionally, any defective filter layer may cause the whole final assembly impractical, thus resulting in a tough requirement of precision and the related environment during the coating process. It is appreciated that for a nearly one hundred filter-layers assembly, significant time and labor are required for fabrication. In conclusion, direct multiple-cavity coating technique is very difficult in manufacturing.
It is also known that the existing multiple-cavity filter device includes a substrate 100 having a filter-film 102 composed of a plurality of filter-layers coated successively on the first side thereof as shown in FIG. 2, and also generally optionally having one anti-reflective film coated on the second side of the substrate. Based on this structure, the commonly light coming from the first side can be filtered through such multiple-cavity filter and reach the second side to be used as a useful light source for the corresponding optical testing or transmission equipment. It should be understood that generally speaking, the filter-layers should be closely stacked with one another without any significant distance between every two adjacent layers, and thus this is the reason why such multiple-cavity filter-layers are only applied to one side of the substrate rather than to both sides of the substrate.
Because it is difficult to fabricate the multiple-cavity filter with large cavity numbers as mentioned before, an object of the invention is to provide a new structure of the multiple-cavity filter device and the corresponding new method for making the same, whereby the manufacturing time and cost can be significantly reduced.